feature/auth-nav

This PR implements the authentication menu in the header. Users that have logged in will have their email address displayed, otherwise they should see "Guest."

Other changes:
- Commented out the user model. At this point, we don't need a custom user model, so just associated the `AUTH_USER_MODEL` with collections.
- Refactored the ordering of model attributes to follow the [django model conventions](https://docs.djangoproject.com/en/dev/internals/contributing/writing-code/coding-style/).
- Changed collection `permissions` attribute to `roles`, since this seems like a better descriptor and will cause less confusion with the built-in django permissions object.
- Modified the URL router to skip the splash page and go straight to the collection index page. We don't currently have a splash page that is useful or functional. We can add this back later if it's deemed necessary.